Vulcanization of elastomers with sulfur has been employed for a long time. Rubber elasticity, resistance to cold flow and improved strength are imparted. The rubber loses its tackiness, becomes insoluble in solvents and is more resistant to deterioration by heat and light. Natural rubber (cis-1,4-polyisoprene) contains one in-chain carbon-carbon double bond per enchained monomer unit, providing more than enough cure sites for sulfur vulcanization. Styrene-butadiene copolymer, a common synthetic rubber, also owes its sulfur vulcanizability to carbon-carbon double bond unsaturation, again present in excess over what is needed for vulcanization.
Saturated elastomers, for example, ethylene-propylene rubbers (EPR) inherently lack carbon-carbon double bond unsaturation and therefore cannot be directly vulcanized with sulfur. An appropriate strategy is to introduce unsaturation by terpolymerization of a few percent of a bifunctional monomer such as 1,5-hexadiene, ethylidenenorbornene or dicyclopentadiene. One double bond of the enchained diene unit survives the terpolymerization process and remains to serve as a cure site for sulfur vulcanization. Alternatively, EPR can be vulcanized with peroxides, a process that is satisfactory with a saturated elastomer and does not require the presence of unsaturation.
EVA copolymer elastomers are also known in the art (I. O. Salyer and H. M. Leeper, Rubber Age, June and July, 1971), and since they too are saturated materials, they are cured with peroxides. Peroxide-cured EVA rubbers possess good thermal stability, good oil resistance and reasonably good low temperature properties. Nevertheless, decomposition products from peroxide curing can cause porosity in the finished product, especially in open or atmospheric pressure cures; other vulcanization systems are therefore desirable. Moreover, rubber formulators very often blend two or more elastomers to achieve a desired balance of properties in the finished product and, as most common elastomers are vulcanized with sulfur, it would be advantageous to have sulfur-vulcanizable EVA elastomers for blending purposes.
Although it is not practiced to any great extent, sulfur vulcanization of EVA elastomers is nevertheless known in the art. A method commonly chosen to impart sulfur vulcanizability to EVA elastomers involves thermal or catalytic deacetylation of the copolymer, with the evolution of acetic acid and the concomitant introduction of sufficient carbon-carbon double bond unsaturation as cure-sites. For example, McAlvey, et al. (U.S. Pat. No. 2,388,169) pyrolyzed EVA at temperatures between 250.degree. and 400.degree. C. at atomospheric pressure in an inert gas or in solution in a solvent. The pyrolyzed copolymer was vulcanized with a typical sulfur vulcanization system. However, according to McAlvey et al., tensile strengths of no better than 700-950 psi were observed for sulfur-vulcanized EVA copolymers containing 60-70 weight percent vinyl acetate, the compositions known to exhibit the best oil resistance (I. O. Salyer and H. M. Leeper, Rubber Age, July, 1971). Moreover, Van Saane et al. (U.S. Pat. No. 3,451,983) disclose that the EVA pyrolysis process of McAlvey et al., performed in the presence of an inert gas at atmospheric pressure or in a solvent, has the drawback that the resulting unsaturated EVA copolymers are also crosslinked to a high degree, and must be decomposed before they can be processed further. Van Saane et al. avoid crosslinking during pyrolysis of EVA by conducting the process in an organic diluent under pressure; e.g., of at least 90 atmospheres of ethylene.
According to Bernhardt et al. (U.S. Pat. No. 4,004,069), when EVA copolymers are deacetylated thermally, or catalytically with a protonic or Lewis acid, under a vacuum of less than about 50 Torr. while continuously removing acetic acid released as in a heated vacuum double screw extruder, the corresponding olefinically unsaturated linear polymers are produced; i.e., crosslinking is avoided. It would appear that the deacetylations done in an extruder by Bernhardt et al. involve more or less shearing of the EVA. There is no disclosure by Bernhardt et al. regarding the influence of stabilizers during deacetylation. Finally, in Ger Offen. 2,413,064, Bernhardt et al. disclose sulfur vulcanization of deacetylated EVA or of blends of deacetylated EVA with other rubbers.
However, in a scientific study [Polymer Letters 11, 521 (1973)] of the thermal deacetylation of an EVA copolymer containing 38.5 weight percent of vinyl acetate, at 260.degree.-290.degree. C., and in a vacuum 10.sup.-4 to 10.sup.-5 mm Hg in sealed ampules with continuous removal of acetic acid by freezing out, Razuvaev et al. found that the copolymer rapidly crosslinks and that the rate of formation of crosslinked material has an autocatalytic character. Razuvaev et al. also found that protonic and Lewis acids accelerate deacetylation and that radical reaction inhibitors (triphenylmethane, or 2,4,6-tritertiary-butyl-phenol) had no effect. It is reasonable to infer that shear was not involved in deacetylations done in sealed ampules by Razuvaev et al.
Deacetylation during the sulfur vulcanization process itself has apparently been accomplished by Miyakawa et al. (Jap. Kokai 75,138,044), who include benzenesulfonic acid, a deacetylation catalyst, in the vulcanization formulation.
In contrast to the above described conventional schemes for sulfur vulcanization for EVA copolymer elastomers in which unsaturation must be introduced before vulcanization can occur, another radically different type of sulfur vulcanization procedure for EVA elastomers is known in the art. Thus, Kaiserman et al. (U.S. Pat. No. 3,972,857) substituted small amounts of .alpha.-chloroacetoxy groups for some of the acetoxy groups of EVA copolymer elastomers by acidolysis of the EVA with chloroacetic acid in xylene solution, removing the corresponding small amount of acetic acid released by azeotropic distillation. The ethylene-vinyl acetate-vinyl chloroacetate terpolymer compositions thus produced by post-polymerization chemical modification were vulcanized by fast soap-sulfur vulcanization systems developed for acrylate ester elastomers, e.g. U.S. Pat. No. 3,458,461. The said acrylate ester elastomers also contain a few percent of vinyl chloroacetate cure sites introduced by copolymerization with vinyl chloracetate. Finally, grafts of acrylate ester containing vinyl chloroacetate cure sites have been made onto EVA elastomers and the resulting grafted polymer products cured with soap-sulfur systems by Chang et al. (U.S. Pat. No. 4,202,845).
The mechanism of soap-sulfur vulcanization of acrylate ester rubbers containing vinyl chloroacetate cure-sites has been studied by Kaendler et al. [Die Angewandte Makromolekulare Chemie 29/30, 241 (1973)]. These authors propose that HCl is eliminated between an .alpha.-hydrogen of an acrylate ester unit in one chain and the active Cl atom of chloroacetoxy group in another chain, with the formation of an S.sub.X bridge between the two carbon atoms. The function of the soap is to neutralize HCl released. Carbon-carbon double bond unsaturation is not invoked in this cure mechanism.